CONFORMATIONAL DYNAMICS OF FLEXIBLY AND SEMIRIGIDLY BRIDGED ELECTRON DONOR-ACCEPTOR SYSTEMS AS REVEALED BY SPECTROTEMPORAL PARAMETERIZATION OF FLUORESCENCE

Conformational changes of intramolecular charge-transfer (CT) species were studied by means of time-resolved fluorescence spectroscopy. From a time-resolved fluorescence spectrum the parameters which describe a kinetic, compartmental model as well as the fluorescence spectral-shape parameters were estimated. The systems (Chart 1) differed in donor and acceptor strengths and possessed two different kinds of bridges: a flexible trimethylene chain and a semirigid piperidine ring. In moderately polar solvents (epsilon > 3.5) only fluorescence originating from an extended dipolar species (extended charge transfer (ECT) state) was found, possessing a dipole moment of about 27 D. In low dielectric media the systems with a Strong cyanonaphthalene acceptor showed ''harpooning'': the ECT species was converted to a folded dipolar species (contact charge transfer (CCT) state), similar to a tight polar exciplex, on a time scale of less than 1 ns (flexible bridge) or 7 ns (semirigid bridge). No indications were found for an ECT in the systems with a weak naphthalene acceptor in nonpolar solvents. Semirigidly bridged systems displayed only local fluorescence, whereas in the flexibly bridged systems CT fluorescence originating from two types of CCT was found-instead.